Resistive and electromagnetic couplers are commonly used to probe signals such as logic signals. A resistive coupler is a device that when operated is in conductive contact with a transmission line to tap a signal passing through the transmission line. (As used herein the term: “tap” refers to acquiring, providing, or otherwise making available a signal off of a transmission line without unreasonably altering the signal. The term: “transmission line” refers to the material medium or structure that forms all or part of a path from one place to another for directing the transmission of one or more electrical signals. For example, a transmission line may comprise wires, traces, contacts, pins, circuit devices, and the like.) An electromagnetic (EM) coupler is a coupler that does not conductively contact a transmission line but instead is suitably positioned next to it to electromagnetically tap a signal in the transmission line.
FIG. 4 schematically shows a resistive coupler arrangement typical of contemporary techniques. A transmitter Tx sends data (e.g., via a gigabit bit stream signal) to a receiver Rx. A resistive coupler 402 couples a sample of the signal to a resistive coupler receiver 406. With such a relatively high frequency signal, the resistance of the resistive coupler 402 is made sufficiently small to obtain a flat sampling of the signal components from the low frequency to the highest frequency components containing discernable energy. The bandwidth and signal to noise ratio constraints typically require that the resistance of the resistive coupler 402 be limited to a range of 200 to 400 ohms. Unfortunately, this can place an unreasonable energy loading on the circuit whose signal is being tapped.
FIG. 5 shows an EM coupled approach to sampling a logic signal arrangement typical of conventional techniques. An EM coupler 504 couples the signal from the transmission line (TL as shown in FIG. 4) to an electromagnetic coupler receiver 508. Termination resistors RT are included for impedance matching the EM circuit in order to enhance electromagnetic coupling. Due to the signal differentiation produced by the EM coupler, the receiver 508 is typically made to integrate the EM coupler signal in order to recover useful information corresponding to the waveform from the transmission line. As compared with a resistive coupler, it generally can be designed to draw less energy from the transmission line signal, but it may not provide lower frequency information, which can be useful in some applications.
Accordingly, an improved coupler approach may be desired.